Deb, that clearance on the wren guard MUST be at least 2 1/2″, and I have learned (from here) that 3″ is better. Probably need more clearance at bottom than at the top if yours is not exactly level. My current nestbox (Gilwood style) has a wren guard with almost 3″ clearance from hole & she is able to swoop right under with no trouble at all. I’m sure this would be why yours is not going in. You need to put some sort of spacer between the box top and the guard to make it extend out more – just unscrew the guard & put in any sort of 1″ spacer & then screw it back in . Apparently your roof is not long enough – I have done this spacer thing several times with success. I, too, use a thin piece of wood (cedar) which extends just below the hole but it must have a minimum clearance of 2 1/2″ and better with 3″. You need to do this as quickly as possible so she will be able to lay the rest of her eggs! Good luck.